The present invention relates to a tunable laser source device employed in evaluating or manufacturing the optical communication system or device.
A configuration of the tunable laser source device in the prior art will be explained with reference to FIG. 4 hereunder.
In FIG. 4, the light emitted from the tunable laser source portion 11 is output to the outside of the tunable laser source device via the optical coupler 18a as the optical output.
Further, the light branched by the optical coupler 18a is branched by the optical coupler 18b. One branched output is fed to the wavelength measuring device 16 that measures the wavelength by utilizing the periodical change of the interference power generated based on the deviation between optical path lengths in the etalon, etc. The other branched output is fed to the gas cell as a reference for callibrating wavelength 15 and the wavelength measuring device 16.
Further, detected outputs of the gas cell as a reference for callibrating wavelength 15 and the wavelength measuring device 16 are converted into electric signals and then fed to the control circuit 14 that is constructed by CPU.
Further, the control circuit 14 controls the wavelength of the light, that is output from the tunable laser source portion 11 via the motor driving circuit 12 and the LD current driving circuit 13, in response to the set signal from the user interface portion 17.
Next, the details of the tunable laser source portion 11 will be explained with reference to FIG. 3 hereunder.
FIG. 3 is a view showing a detailed configuration of the tunable laser source portion 11. This configuration includes the semiconductor laser (LD) 21, the lenses 22a, 22b, the diffraction grating 23, the mirror 24, and the motor 25.
The light emitted from the semiconductor laser 21 is shaped into the parallel light by the lens 22a and then enters into the diffraction grating 23.
Only the light having the wavelength, which is decided by the positional relationship between the diffraction grating 23 and the mirror 24, out of the light incident into the diffraction grating 23 can be fed back to the semiconductor laser 2l once again. As a result, the light having the particular wavelength is output from the semiconductor laser 21 via the lens 22b. 
If the external cavity length is changed by driving the motor 25 to rotationally move the position of the mirror 24 around the center O of rotation, the wavelength of this output light can be changed.
In this case, if the motor 25 is set simply to a predetermined position, sometimes the infinitesimal error is generated in the position of this mirror 24. Therefore, as shown in FIG. 4, the detected output of the wavelength measuring device 16 is fed back to the control circuit 14 such that the control is carried out by driving the motor 25 to mate always the measured wavelength with the predetermined wavelength.
Further, the wavelength of the light generated by the semiconductor laser can be controlled by adjusting the driving current of the semiconductor laser. Therefore, the wavelength of the light can be controlled by feeding back the detected output of the wavelength measuring device to the driving circuit of the semiconductor laser.
As described above, the wavelength measuring device that measures the wavelength by utilizing the periodical change of the interference power based on the deviation between optical path lengths in the etalon, etc. is employed as the wavelength measuring device in the tunable laser source device in FIG. 4. Therefore, in order to compensate the change in the deviation between the optical path lengths due to the change of the ambient temperature, the temperature controlling device for maintaining the wavelength measuring device at the constant temperature (temperature control) is provided.
Further, the gas cell as a reference for callibrating wavelength 15 is provided to the tunable laser source device in FIG. 4 and is used to calibrate the wavelength measuring device.
In this case, the gas cell as a reference for callibrating wavelength 15 in FIG. 4 looks for the wavelength at one point from already-known absorbed line wavelengths as the reference wavelength.
This already-known wavelength is set as the reference wavelength of the wavelength measuring device 16 in which, as shown in FIG. 5, the periodical change of the interference power is present.
As shown in FIG. 5, the wavelength linearity correction table by which the output of the wavelength measuring device 16 in which the periodical change of the interference power is present is corrected on the basis of the reference wavelength is formulated, and then is stored in a memory means (not shown) in the control circuit 14.
As shown in FIG. 5, the gas cell 15 has the characteristic to absorb the wavelength at the particular already-known one point. The absorbed line wavelengths of the gas cell are very stable to the environmental change such as the change of the ambient temperature, and others.
In contrast, as shown in FIG. 5, the measured output of the wavelength measuring device 16 that utilizes the deviation between the optical path lengths changes to have peaks and notches of the power periodically.
However, the interval between the peak (notch) and the peak (notch) has the characteristic that depends on the change of the ambient temperature.
More particularly, as shown in FIG. 5, even though the wavelength linearity correction table is formulated at a certain ambient temperature while employing the absorbed line wavelength, that is indicated by an arrow, of the gas cell as a reference for callibrating wavelength 15 as the reference value of the measured output of the wavelength measuring device 16, such measured output of the wavelength measuring device 16 in FIG. 5 is expanded and contracted in the lateral axis direction if the ambient temperature is changed. As a result, the error is generated in the wavelength linearity correction table.
Accordingly, there exists the following problem in the tunable laser source device set forth in FIG. 4 in the prior art.
Although the temperature control is applied to the wavelength measuring device, the infinitesimal temperature change is caused in the device if the ambient temperature of the device in formulating the wavelength linearity correction table at the time when the device is carried out of the factory is different from that of the device in user""s employment. For this reason, such temperature change exerts not a little effect upon the wavelength measuring accuracy.
In order to maintain the wavelength accuracy obtained at the time when the device is carried out of the factory against the variation in the atmospheric temperature, the wavelength measuring device must be installed into the high performance temperature controlling mechanism (thermostatic bath). Normally, these high performance thermostatic baths are large in size and high in cost.
It is an object of the present invention to overcome the problems of the tunable laser source device set forth in FIG. 4 in the prior art, more particularly the problem such that, when an ambient temperature of the device in user""s employment becomes different from that of the device in preparing a wavelength linearity correction table at the time when the device is carried out of a factory, a minute temperature change is caused in the device to have not a little effect on a wavelength measuring accuracy.
In order to overcome the above subjects, there is provided a tunable laser source device for branching a light output from a tunable laser source portion to supply to a wavelength measuring device and a wavelength calibrating reference device and then controlling the tunable laser source portion in response to an output of the wavelength measuring device,
wherein at least one peak and one notch or two peaks or two notches or more in a measurement interference period of the wavelength measuring device are included between a plurality of reference wavelengths of the wavelength calibrating reference device. (Aspect 1)
According to this, since at least two points, i.e., one peak and one notch in the measurement interference period of the wavelength measuring device can be calibrated by the wavelength calibrating reference device, the precise wavelength measurement can be achieved even if the external environment is changed.
Further, the wavelength calibrating reference device is formed of a gas cell. (Aspect 2)
Further, the wavelength measuring device measures the wavelength by utilizing the periodical change of the interference power based on the deviation between optical path lengths. (Aspect 3)
Further, the wavelength measuring device can be formed of an etalon. (Aspect 4)
Further, if the tunable laser source portion is constructed such that it can sweep continuously a wavelength thereof, the effect of improving the wavelength accuracy in the continuous wavelength sweep can be increased. (Aspect 5)
These tunable laser source devices that are capable of sweeping the wavelength continuously with high accuracy can contribute the event that the accuracy in the measurement of the wavelength dependency characteristic of optical parts employed in the optical communication, etc. is increased considerably.